Yep, I'm surprised the OP doesn't use such a scheme, possibly with something SSE-ish so you don't mix the content and comments on-disk but they're both retrieve

Yep, I'm surprised the OP doesn't use such a scheme, possibly with something SSE-ish so you don't mix the content and comments on-disk but they're both retrieved as a single request. This way all you need is a POST endpoint (which might just be a cgi script) to receive the comment and update the comment file. Gets more complicated if you support threaded comments or pagination, but for a linear comment system it should be more than enough.